1. Field of the Invention
This invention relates to an extrusion die for a blow head for making blown plastic films, which die comprises inner and outer rings, which are made of a ferrous material, particularly steel, and define between them an annular orifice.
2. Description of the Prior Art
Because the molten plastic materials flows along paths of different lengths from the extruder screw to the extrusion die or because other inhomogeneities occur, a non-uniform temperature profile around the periphery of the extruded parison may be obtained at the outlet end of the die orifice and may result in the blown plastic film in thickness variations which are out of tolerance.
In an extrusion die of the kind described first hereinbefore, which has been disclosed in German Patent Publication No. 21 40 194, the outer ring is divided into a plurality of temperature control segments, which are provided with separate temperature control chambers, which are connected to separate inlet and outlet lines for the heating or cooling fluid. The occurrence of zones which are at different temperature in the molten plastic material owing to different flow rates and flow paths of said material can be avoided if the feeding of the heating or cooling fluid to the temperature control chambers is properly controlled.
Published German Application No. 32 11 833 discloses an extrusion die which is of the kind described first hereinbefore and in which the inner and outer rings are formed with annular gaps, which are filled in part with a heat exchange fluid that is adapted to be evaporated and re-condensed at temperatures in the operating temperature range of the blow head so that temperature variations occurring over the height and/or periphery of the annular gap can be compensated in that the heat exchange fluid evaporates in relatively hot portions of the annular gap and re-condenses in relatively cold portions.